A signal sent by a transmitter entity suffers distortion as a function of propagation conditions and in particular the power of the signal received by a receiver varies in time and in space. In a radio telecommunications network, a transmitter entity is for example a radio transmitter including a transmitter antenna, a radio base station, or a fixed or mobile radio relay station including a device for sending a signal coming from a radio base station. A transmitter entity may equally be a transmitter of signals by acoustic means in an undersea medium or a transmitter for a fixed telecommunications network, for example an optical fiber network or a copper network.
In order to improve the quality of the signal received by a receiver, replicas of the signal may also be transmitted to the receiver. Temporal diversity is obtained in this way by transmitting the signal and replicas with a time shift, spatial diversity is obtained by transmitting the signal and replicas via different antennas, and frequency diversity is obtained by transmitting the signal and replicas on different frequencies.
The spatial and temporal diversity properties are thus exploited in MISO (multiple input single output) telecommunications networks in which different replicas of the signal are transmitted by colocated transmitter entities.
The replicas of the signal are generated by applying a space-time code to the temporal signal to be sent consisting of a succession of symbols. The space-time code is selected as a function of the properties of temporal orthogonality between the signal and the replicas obtained after space-time coding. The selection of the code depends in particular on the number of transmitter entities. With two colocated transmitter entities, it is routine to use an Alamouti code as described in the document “A simple transmit diversity technique for wireless communications” by S. M. Alamouti published in IEEE Journal on Select Areas in Communications, vol. 16, 1998. The replica of the signal obtained after applying the Alamouti code consists of a succession of symbols orthogonal to the succession of symbols before coding.
Using these space-time codes thus makes it possible to exploit the spatial and temporal diversities obtained by transmitting from the same location the signal and one or more orthogonal replicas of the signal, the signal and the replicas suffering similar distortion between the various transmitter entities and the receiver, and notably suffering identical attenuation of their power and being subject to the same propagation delay.
However, for distributed MISO telecommunications networks in which the various replicas are transmitted by different and geographically remote transmitter entities, the properties of temporal orthogonality between the signals and the replicas are no longer respected at the receiver level. The propagation conditions between each transmitter entity and the receiver are different, notably the propagation time to which the signal and the replicas are subjected. Thus the signal and the replicas are subjected to different time shifts that may prove impossible to compensate on transmission and the properties of orthogonality between the signal and the replicas are no longer respected at the receiver level. Space-time decoding in the receiver is then no longer possible.
The document “System Performance of Distributed Transmit Diversity of OFDL-based 1xEV-DO Broadcast Network” by Young C. Yoon, Alpaslan Savas, and Wanshi Chen, published in the proceedings of the Wireless Communications and Networking Conference, IEEE-WCNC 2006, vol. 3, pages 1475-1479, applies the principle of the orthogonality of the Alamouti code in the frequency domain to a distributed MISO-type network. Thus an OFDM (orthogonal frequency division multiplex) signal and an orthogonal replica of the signal in the frequency domain generated by applying an Alamouti code to the succession of symbols to be sent are transmitted to a receiver by two geographically-remote transmission entities. Any time shift being reflected in a phase-shift in the frequency domain, a time shift between a signal and its replica therefore no longer affects Alamouti decoding in the receiver, by means of the frequency processing carried out to constitute the OFDM signals on transmission and ODFM demodulation on reception. However, this is possible only for a distributed MISO network sending a signal and a single replica to a receiver from two geographically-remote transmitter entities. This restriction prevents deployment of distributed MISO telecommunications networks sending a signal by a plurality of transmitter entities, for example in so-called collaborative telecommunications networks.